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Subject: Re: [linux-sunxi] Re: [PATCH v3 6/7] arm64: allwinner: h6: add the
 basical Allwinner H6 DTSI file
To: samuel@sholland.org, Maxime Ripard <maxime.ripard@bootlin.com>,
        Icenowy Zheng <icenowy@aosc.io>
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From: Andre Przywara <andre.przywara@arm.com>
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Date: Wed, 28 Feb 2018 11:08:07 +0000
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Hi,

On 26/02/18 15:54, Samuel Holland wrote:
> On 02/26/18 03:26, Maxime Ripard wrote:
>> On Fri, Feb 23, 2018 at 11:22:06PM +0800, Icenowy Zheng wrote:
>>>>> +	psci {
>>>>> +		compatible = "arm,psci-0.2";
>>>>> +		method = "smc";
>>>>> +	};
>>>>
>>>> Is it needed? The bootloader should fill it with whatever version it
>>>> has, shouldn't it?
>>>
>>> But we now use ATF rather than U-Boot PSCI. U-Boot will not fill ATF
>>> info.
>>>
>>> See A64/H5 device trees.
>>
>> So if the PSCI version implemented in ATF ever changes, we would have
>> to update all the DT everywhere, but only if you're running the new
>> version?
> 
> Yes but no. PSCI 1.0 is generally backward compatible with PSCI 0.2. In fact,
> the Linux driver treats them exactly the same:
> 
>         { .compatible = "arm,psci-0.2", .data = psci_0_2_init},
>         { .compatible = "arm,psci-1.0", .data = psci_0_2_init},

Yeah, it's actually unclear why we have that new compatible name in the
first place, since starting with PSCI v0.2 the version can be perfectly
read using the mandatory PSCI_VERSION call. And DT aims to only cover
things that are not discoverable.

> For the H6, however, the oldest ATF source available (which I believe was the
> one in use during bringup) is based on mainline 1.4, and is already at PSCI
> version 1.1:
> 
> [    0.000000] psci: probing for conduit method from DT.
> [    0.000000] psci: PSCIv1.1 detected in firmware.
> [    0.000000] psci: Using standard PSCI v0.2 function IDs
> [    0.000000] psci: MIGRATE_INFO_TYPE not supported.
> 
> So we could go ahead and bump the compatible to "arm,psci-1.0".

I'd rather leave it at "arm,psci-0.2", since this improves compatibility
to other OSes or DT users.

Cheers,
Andre.